69 AMX vacuum wipers

The windshield wipers on my AMX suddenly stopped returning. They come up and then start to return for the next up sweep, but stop after a few inches. The wiper transmission moves manually with ease. I can hear a vacuum leak when the switch is fully open. Turning the switch slightly will make the leak cease but wipers still won’t return for the up swipe. The slider on top of the motor also moves fully with the switch. I took the top cover off the wiper motor ( the sliding part now exposed) and saw a rubber block encased in plastic. Didn’t look dirty but wiped it with some alcohol anyway. It did not help. Anyone else experience this to give me some pointers?   I fear I may have to remove the wiper motor for rebuilding.  Not sure what to do next. Thanks for help. 

The clip that goes on the wiper motor shaft to transmission? I usually have the end of the wiper transmission sticking out of the cowl hole where thw motor mounts. Then put the arm off the trans on the motor and clip it. Then slide the motor into place and secure it. 

Lay the linkage in the cowl with attached pull strings up through the holes for the posts. Attach the linkage clip. Then pull posts through the holes with the strings.